One of the main problems with classical periodograms is spectral leakage, of which there are several forms. Leakage to nearby frequencies (sidelobes) is due tio the finite total interval over which the data has been sampled. Leakage to distant frequencies is due to the finite size of the interval between the samples.
Examining the Periodogram Analysis popup generated from the Operations=>Find Periodicties item in the plot window we see an option to Generate Window Function. This option sets all the y-axis data points to unity. A discrete Fourier transform of teh resulting data yields the window function (or spectrum), which shows the effects of spectral leakage.
FROG - A Time Series Analysis Package